Abstract: Using RETRAN-02, the paper researches systematically the ways of lacalboiling startup of 5 MW Low Temperature Heating Reactor (LTHR), designed andbuilt at Institute of Nuclear Energy and Technology (INET) of TsinghuaUniversity. The affect of startup stability by the rate of control reactivityinsertion, the amount of vapour condensation of the primary collant system andthe amount of heat transfer of the primary coolant system are analysed. Theresult shows that, by a new type of mode of startup, the reactor can transitstably from single phase natural cycle to two-phase local boiling natural cycle mode,as long as some special parameters of the reactor are controled temporarily.